[Runtime] Document registering your own runtime template

Author: theofidry

components/runtime.rst

@@ -101,6 +101,26 @@ Use the ``APP_RUNTIME`` environment variable or by specifying the
         }
     }
 
+If modifying the runtime class is not enough, you can always create your own runtime
+template:
+
+.. code-block:: json
+
+    {
+        "require": {
+            "...": "..."
+        },
+        "extra": {
+            "runtime": {
+                "autoload_template": "resources/runtime/autoload_runtime.template"
+            }
+        }
+    }
+
+If you want a reference, the The Symfony's runtime can be found at
+<a href="https://github.com/symfony/symfony/blob/5.4/src/Symfony/Component/Runtime/Internal/autoload_runtime.template" class="reference external" title="Symfony autoload_runtime.template" rel="external noopener noreferrer" target="_blank">src/Symfony/Component/Runtime/Internal/autoload_runtime.template()</a>.
+
+
 Using the Runtime
 -----------------